1. Field of the Invention
This invention relates to isocyanate compositions and is more particularly concerned with color stabilized organic isocyanates.
2. Description of the Prior Art
The tendency of many organic isocyanates to form undesirable colored impurities on storage is well-known. This is especially true in the case of those compounds which have isocyanate groups attached to aromatic rings which rings are connected by methylene groups. Methylenebis (phenyl isocyanate) is typical of such isocyanates and is a compound which is originally white when obtained by distillation but which, on storage at ambient temperatures or lower, even in the absence of oxygen and sunlight, frequently becomes colored yellow or brown and even green or blue in certain instances. The same phenomenon is observed in the case of various forms of this diisocyanate, such as liquid prepolymers thereof derived by reacting the diisocyanate with a minor amount of an aliphatic diol or mixture of such diols.
Numerous attempts to cure the above problems by the addition of antioxidants and the like have been reported and have met with varying degrees of success. Typical are the processes reported in U.S. Pat. Nos. 2,885,420-6; 2,950,307; 3,585,229 and 3,715,381. We have found, however, that, under certain circumstances, methylenebis(phenyl isocyanates) and related compounds can develop coloration on storage at ambient temperatures even in the presence of relatively large amounts of the antioxidants and other materials employed in the prior art to prevent color formation. We have also found that the prior art antioxidants and additives are not effective in discharging color which has already been formed by impurities in said diisocyanates even though the same additives can inhibit the formation of said coloration if added to the diisocyanate before the coloration has been allowed to occur.
The present invention is based on our discovery that certain alicyclic and heterocyclic compounds containing the grouping ##STR2## are highly effective in preventing discoloration on storage at ambient temperatures of organic polyisocyanates susceptible to coloration on storage and particularly in respect of methylenebis (phenyl isocyanates) and related compounds. These same alicyclic and heterocyclic compounds can also be used to discharge coloration in said isocyanates after the formation of colored impurities has taken place. Certain of the compounds which are utilized in accordance with the present invention have been reported as useful in catalyzing the reaction between isocyanates and active-hydrogen containing compounds (see U.S. Pat. No. 3,914,189) and in the preparation of isocyanate-terminated prepolymers in the formation of moisture-curable coating compositions (see U.S. Pat. No. 3,661,861). However, it has not been reported previously that very minor amounts of said compounds can be used to stabilize isocyanates against color formation or to discharge color from isocyanates containing colored impurities.